The Editor’s Choice for our May issue is “Herbivore dung stoichiometry drives competition between savanna trees and grasses” by Judith Sitters & Harry Olde Venterink. This article shows that browsing and grazing herbivores potentially help maintain the tree‐grass balance in African savanna, through variation in the nitrogen (N) to phosphorus (P) ratio of their dung!
Here the handling Associate Editor, Carla Staver, discusses the importance of this article, which advances our understanding of tree and grass niche differences in savanna ecosystems.
Savanna ecosystems are co-dominated by trees and grasses, which are pretty distinct in terms of their growth form and physiology. Stoichiometrically, grass often has less N in leaves that trees relative to P, with implications for its palatability as well as for herbivore-driven nutrient cycling. As it turns out, grazers (which eat more grass) produce dung with a small N:P ratio than browsers (which eat more trees).
Left: Giraffe (Giraffa camelopardalis). Right: Burchell’s zebra (Equus quagga burchellii).
The question that this paper asks is whether this feeds back to impact tree and grass growth and competition. To do this, this paper elegantly weaves together a couple of interesting threads from disparate fields of ecology. Firstly, biogeochemists have long hypothesized that nitrogen-fixing plants take up less nitrogen from soils than non-fixers do, relying instead on nutrients like phosphorus which are required in large quantities to fuel N-fixation. This has been well studied in tropical forests and, classically, on Hawaiian chronosequences, but has received much less attention in African savannas, despite a large diversity and relative dominance of acacia trees with the capacity to fix nitrogen. Separately, savanna ecologists have often considered how it is that trees and grasses can coexist in a system that is clearly resource limited and especially what role resource niche specialization might play in promoting tree-grass coexistence. However, water has been the dominant focus of this work, and a possible role for nutrients in differentiating tree and grass niches is much less clear. Finally, savannas are also systems where fire and herbivory impose strong constraints on vegetation, independently and in interaction with resource limitation. However, these separate determinants of vegetation dynamics are rarely integrated for a synthetic perspective on tree-grass dynamics in savannas. This paper pulls together all three areas for a nuanced and original take on tree-interactions.
In this article, Sitters & Olde Venterink collected dung from a grazer (zebra) and a browser (giraffe) and used it to fertilize a greenhouse experiment in which they competed trees and grasses. They found that N-fixing trees were favored by grazer dung (relatively rich in P), whereas grasses were favored by browser dung (rich in N). In simple terms, dung composed of grass tended to favor trees, whereas dung composed of tree foliage tended to favor grasses. This, in effect, established a stabilizing feedback. Too many N-fixing trees in the landscape might favor grasses (by tipping the balance towards plentiful N), while too much grass might create an advantage for N-fixing trees.
This is an elegant and cool result, which advances our concept of how tree and grass niches are different in savanna systems. There are open questions, of course. I personally am curious about how trees that can’t fix nitrogen fit into the mix – do they behave more like grasses or like trees in competition for N and P, and how does this factor into tree-grass dynamics more broadly? And how does the finding that grasses are highly N-demanding fit with the observation that fires volatilize so much N, impoverishing systems that might otherwise be richer in N? From an applied perspective, in a world where anthropogenic N-deposition is widespread and substantial, how have changing biogeochemical cycles changed the interactions between trees and grasses? This should be a rich area of research, with intriguing options for future study.
Carla Staver Associate Editor, Journal of Ecology
You can read the full article by Sitters & Olde Venterink here: Herbivore dung stoichiometry drives competition between savanna trees and grasses